1. Technical Field
This invention relates to tuneable radio antennas having loading coils for impedance matching to a co-axial feed line.
2. Description of Prior Art
Prior art antenna radiating elements are typically matched to feed lines by using a tapped loading coil at the feed point of the co-axial feed line. Prior art devices have used a number of different approaches to tune such antennas given the nature of the selective structures. One variation is to couple the ground end of the loading coil through a variable capacitor to the ground plane, such as a mechanical body. Tuning is then achieved by rotating a variable capacitor depicting a series resonant tuning circuit.
An alternate method is described as parallel resonant tuning circuit in which a variable capacitor is connected between the antenna radiating element (beyond the loading coil) and ground. The variable capacitor is rotated to tune the antenna. These prior art attempts to use capacitance means for tuning are typically complex and require adjusting capacitances by hand during operation.
Prior art examples of the tuning methods described above can be seen, for example, in U.S. Pat. Nos. 3,541,554, 3,798,354, and 4,080,604.
In U.S. Pat. No. 3,541,554 a tuneable whip antenna is disclosed using a pair of inductant coil in series between the load in cable and radiating element. A ring is peripherally disposed thereabout for adjustably varying the mutual inductance between the coils.
U.S. Pat. No. 3,798,654 is directed to a tuneable sleeve on a radiating element with a resonant tuning coil coupled thereto and electrically coupled tuning element.
A means for tuning a loaded coil antenna is disclosed in U.S. Pat. No. 4,080,604 wherein a loading coil has a conductive member opposite several turns of the loading coil and is disposed in distributive capacitive relationship therewith. The conductive member is movable along the coil surface to change the effective resonant frequency of the antenna.